The present invention relates to a method and apparatus for decomposing a semiconductor thin film, i.e., a thin film formed on a semiconductor substrate such as a gate oxide film and, particularly, to a method and apparatus which permit improved accuracy in the analysis of ultratrace impurities contained in a semiconductor thin film.
As known well, a semiconductor thin film such as an oxide film is formed first on a semiconductor substrate consisting of, for example, silicon in the manufacture of MOSIC or MOSLSI. Then, source and drain regions are formed in the substrate by impurity diffusion, followed by forming a gate oxide film on the gate region.
Since the gate oxide film has serious effects on the electric properties of the produced element, it is necessary to pay careful attention to the processes both before and after the gate oxide film formation. It should be noted in particular that even an ultratrace impurity contained in the gate oxide film such as Na, K or Fe markedly deteriorates, for example, the breakdown voltage of the element. It should also be noted that LSI devices tend to be made finer and finer recently in accordance with increase in the degree of integration, resulting in a serious demand for increased reliability in the breakdown characteristics of the gate oxide film. Naturally, it is a matter of serious concern in this field to decrease the impurity concentration, e.g., the metal impurity concentration, of the semiconductor thin film such as a gate oxide film as much as possible so as to improve the element characteristics. Also, in the supervision of the manufacturing process of MOS devices, it is of great importance to accurately analyze the ultratrace impurities contained in the semiconductor thin film.
It was customary in the past to employ a vapor phase decomposition method, which is a kind of etching technique, for analyzing the ultratrace impurities. Specifically, an oxide film is formed on a semiconductor wafer in a heating furnace or an oxide film-forming apparatus similar to that used in the ordinary manufacture of semiconductor devices. The wafer having an oxide film formed thereon is transferred into a hermetic vessel, and a hydrofluoric acid vapor of extremely high purity is applied to the oxide film of the wafer within the vessel so as to decompose the oxide film and recover a solution of the decomposed oxide film. The recovered solution is stirred and then weighed, followed by applying a known frameless atomic absorption analysis to the solution so as to carry out the quantitative and qualitative analyses of the ultratrace impurities contained in the oxide film. To be more specific, the recovered solution is poured into a graphite furnace with a micropipette and then electrically heated so as to dissociate the decomposed materials into the atomic state. Further, a resonant emission ray of the element to be measured is projected onto the resultant atomic vapor with a hollow cathode lamp, and the absorption intensity thereof is measured so as to determine the impurity concentration.
In the conventional analysis outlined above, it is possible to condense the decomposed solution of the oxide film, leading to a high sensitivity and a high accuracy of the analysis.
However, the conventional method described above is defective in that wafer contamination is caused in the step of forming the oxide film (semiconductor thin film) before the analytical step. Also, the wafer and the hermetic vessel tend to be contaminated in the step of transferring the oxide film-bearing wafer into the hermetic vessel. It follows that the recovered solution of the oxide film is caused to contain impurities providing secondary contaminants, resulting in failure to accurately analyze the ultratrace impurities contained in the oxide film (semiconductor thin film) itself.
To be more specific, the impurity analysis accuracy of the conventional method described above is as low as about .+-.20-30%, failing to fully utilize the high accuracy, e.g., .+-.1% for Na and .+-.2% for Fe, achieved by the frameless atomic absorption analytic apparatus. In other words, the conventional method described above fails to effectively control the impurity concentration of the gate oxide film in the manufacture of MOS devices, leading to a low reliability of the element characteristics.
The problems described above also remain unsolved in the analysis of the ultratrace impurities contained in the semiconductor thin film other than an oxide film, e.g., the ultratrace impurities contained in a nitride film.